The present invention relates to a waveform generator, which generates musical tone waveforms for an electronic musical instrument.
Various methods or apparatuses for generating musical tone waveforms are known. Such methods or apparatuses include:
(A) A method or apparatus for storing a waveform corresponding to each tone color in a memory, and reading it out. More specifically, such a method or apparatus is described in U.S. Pat. No. 3,639,913 (issued on Feb. 1, 1972), U.S. Pat. No. 3,882,751 (issued on May 13, 1975), and the like.
(B) A method or apparatus for multiplying a sine wave of a fundamental frequency and a sine wave of each harmonic component with a Fourier coefficient and synthesizing resultant products to produce a waveform in accordance with a Fourier synthesis method, i.e., ##EQU1## anain n.omega..sub.0 t (where .omega..sub.0 is a fundamental frequency. Such a method or apparatus is described in U.S. Pat. No. 3,809,786 (issued on May 7, 1974).
With method or apparatus (A), various waveforms cannot be stored in a memory unless the memory has a large storage capacity. More specifically, the storage capacity poses a problem. In addition, waveforms other than those stored in the waveform memory cannot be produced.
With method or apparatus (B), in order to obtain a musical tone having desired harmonic overtone components, calculations must be performed a plurality of times corresponding to the number of harmonic overtone components, or sine wave generators corresponding in number to the harmonic overtone components must be required.
In addition, the following method or apparatus is known.
(C) A method or apparatus in accordance with FM modulation.
More specifically, waveforms are produced in accordance with the following equation: EQU Y2=Asin(.omega..sub.c t+Isin.omega..sub.m t)
(where .omega..sub.c is a carrier angular frequency, .omega..sub.m is an angular frequency of a modulated wave, I is a modulation index, and A is an amplitude.) Such a method or apparatus is disclosed in U.S. Pat . No. 4,018,121 (issued on Apr. 19, 1977).
With this FM modulation method, waveforms including many harmonic overtone components can be generated by a relatively small sine wave generating system. However, when resultant waveforms are evaluated in terms of their spectra, a waveform with a moderate spectrum envelope cannot be generated, and a waveform string whose spectrum distribution gradually changes, cannot be generated.
(D) A method or apparatus in accordance with a PD (phase distortion) method.
This method or apparatus relates to a content previously proposed by the present assignee (e.g., U.S. Ser. No. 788,669 filed on Oct. 17, 1985). With this method, phase angle signals for reading a sine wave stored in a single musical tone waveform memory are modified by a phase angle modifying circuit, and various waveforms such as a sawtooth wave and a sine wave can be generated.
However, in a detailed circuit arrangement for method (D), the phase angle modifying circuit is constituted by dividers, resulting in a bulky apparatus.